Compositions of decellularised tissues from warm-blooded vertebrates, including humans, can be used as tissue graft materials. Common tissue graft compositions may be derived from the dermis, the small intestine, the urinary bladder, renal capsule, the simple glandular stomach and the forestomach matrix (see, for example, U.S. Pat. Nos. 4,902,508, 5,554,389, 6,099,567, 7,087,089, and 8,415,159, the entire contents of which are incorporated herein by reference). These compositions are known as extracellular matrix (ECM) and have an important role in providing the optimal chemical and structural environment for tissue growth and regeneration. ECM scaffolds used for tissue regeneration are traditionally prepared from decellularised human and animal tissues isolated from various organs and from a variety of animal connective tissue and basement membrane sources. These scaffolds promote tissue regeneration and are well-tolerated immunologically.
The ideal tissue graft is one that is the closest possible analogue to native tissue. Tissue processing is required to remove cellular components that otherwise may cause rejection and to ensure safety from transmissible diseases. Further processing may be introduced to customise the fabrication of the graft to meet site specific requirements and to improve shelf life. Each successive processing step has the potential to damage the ECM, alter the immune response and consequently affect the tissue remodelling process. Chemical processing, drying and sterilisation techniques damage ECM and therefore affect the in vivo behaviour of grafts (1-3). Ready to use products are favoured by surgeons. Consequently, minimally processed wet grafts are preferable.
One limitation of some ECM graft materials is that the thickness of the graft is determined by the thickness of the tissue layer obtained from the source material. For example, the thickness of forestomach ECM sheet is limited by the thickness of the source tissue. Yet applications such as hernia repair, skin graft, dural replacement, tendon repair and reconstructive surgery often require thicker grafts to provide adequate tensile strength, biomechanical performance and biological activity.
Individual sheets of ECM tissue typically have anisotropic mechanical properties that are directionally specified by the orientation of collagen fibres within the tissue. This results in directional variability in the physical properties of native single sheets of ECM tissue. Laminated graft constructs with alternate sheets having different fibre orientations can minimise the directional variability of the construct, and result in isotropic constructs which are stronger in multiple directions.
To customise ECM grafts to meet site specific requirements it is possible to fabricate laminated graft products that comprise multiple sheets of ECM using techniques such as compression and drying, chemical cross-linking, suturing, or through the use of adhesives (see U.S. Pat. Nos. 5,885,619, 5,955,110, and 8,415,159).
Graft products made using these techniques have limitations. Where air, heat or lyophilisation is used to dry the graft products, the ECM is damaged as a consequence of water loss. In the case of lyophilisation, ice crystal formation leads to structural changes of the scaffold (4). While dehydration and compression have been used as a method to fabricate laminated graft products, the ECM proteins are typically damaged in the process and consequently elicit an exaggerated immunological response. A further limitation of dehydrated and compressed products is their tendency to delaminate after rehydration, during surgical handling, implantation and subsequently over time.
Sutures may be used to overcome the tendency of laminated graft products to delaminate. However, sutures introduce a foreign material into the graft and this can lead to inflammation, scarring and encapsulation, and may not be desirable in some situations (5). For example, fabricating a laminated ECM graft product using permanent synthetic sutures, such as Prolene, to secure the sheets together will result in the replacement of the ECM over time but the synthetic sutures will not be remodelled. This is not desirable in applications where the graft should be completely replaced by the patient's own tissue. Permanent sutures result in a greater likelihood of infection and limit the utility of this type of product in open dermal repair applications. Graft products that include absorbable sutures have a limited shelf-life in a wet presentation because the sutures often break down through hydrolysis (6,7). Consequently, in applications that require a high tensile strength over a sustained period of time, absorbable sutures are not suitable.
Laminated graft products may also be comprised of ECM sheets held together using an adhesive. However, introducing an adhesive can create a barrier to cell migration and can alter the typical mechanisms and kinetics of ECM remodelling.
Graft products that comprise chemically cross-linked sheets of ECM bound together are known to elicit a foreign body response and have limited biotrophic properties.
Graft products that are exclusively comprised of a synthetic polymer mesh are known to provide long-term strength and rigidity. However, over time the large amount of synthetic material can cause a foreign body response and may result in mesh erosion where the mesh can pass through layers of tissue. Synthetic polymer material grafts have no biological component and therefore do not provide biological assistance with wound and tissue repair. This can lead to encapsulation of the graft and increases the risk of adverse reactions.
It can therefore be seen from the problems and disadvantages associated with existing graft products that there is a need for laminated graft products that comprise ECM and/or a natural or synthetic polymer material and can be readily tuned to meet the biophysical requirements of a wide range of anatomical sites and do not delaminate during surgery or after implantation. Furthermore, a graft product that remains intact in both dry and wet presentations is desirable. A laminated graft product that can be presented in a wet form to avoid damage to the ECM and retain inherent biotrophic properties is most desirable.
It is therefore an object of the invention to provide a tissue graft product comprising two or more layers of ECM or polymeric material which overcomes, at least in part, one or more of the abovementioned problems, or to at least provide a useful alternative to existing products or procedures.